Nebojša Tower
Belgrade, Serbia
Nebojša Tower (Serbian, Kula Nebojša) is the only surviving mediaeval tower of the Belgrade Fortress. Built in the 15th century, it was the major defensive tower of the fortress for centuries. Later it served as a dungeon and in 2010 it was adapted into a museum. The tower is located near the confluence of the Sava into the Danube.
Prison data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).